The germline is the basis for totipotency and persistence and diversity of a multicellular organism are imparted by transmitting genetic information and epigenetic information to the next generation in many multicellular organisms. When an abnormality occurs in the germ line in which human primordial germ cells (PGCs) develop and form spermatozoon or ovum via a complicated and diversified developmental pathway, various developmental disorders such as genetic or acquired diseases and developmental disorders, functional disorders, infertility and the like appear in the offsprings. Therefore, understanding of the mechanism of germ cell formation is very important for the understanding of not only biology but also disease.
The mechanism of gametogenesis in mammals has been studied in mice and many findings were obtained. They provide important information possibly applicable to all animals including human. However, in variously existing animal species, elaborate mechanisms concerning germ cell development vary markedly among species. For accurate understanding in each species, findings in each animal species are considered to be necessary.
Information on the mechanism of germ cell development in human is very lacking. This is caused by the difficulty in obtaining experiment materials. As described above, since the mechanism of human germ cell formation is unknown, diagnosis/treatment of diseases caused by defects in human germ cells has been difficult.
Breakthrough is said to occur by reconstituting the development of human germ cells in vitro using human pluripotent stem cells (hPSCs) such as human embryonic stem cells (hESCs) (non-patent document 1) and human induced pluripotent stem cells (hiPSCs) (non-patent document 2).
Induction from pluripotent stem cells (PSCs) to mouse germline and the development thereafter have been reproduced in vitro (non-patent document 3 and non-patent document 4). That is, recent studies have clarified that mouse (m) with pluripotency of ground state ESCs/iPSCs (non-patent document 5) is induced to pre-gastrulation ectoderm-like cells (EpiLCs), and successively to PGC-like cells (PGCLCs) having epigenetic properties extremely similar to migratory PGCs and global transcription. Surprisingly, PGCLCs induced in this manner have high capacity in both the formation of spermatozoon and ovum and progeny generation, and these facts propose a conceptual framework for the reconstruction of human germ cell development in vitro and suggest that such reconstruction can be realized.
However, hESCs/iPSCs are different from mESCs/iPSCs in terms of gene expression profile, epigenetic property, cytokine dependency, and differentiation potency, and are considered to be in pluripotency state similar to mouse ectoderm stem cells (epiblast stem cells (EpiSCs)) (non-patent document 6 and non-patent document 7). Such state is similar to post-gastrulation mouse epiblast, which means that capacity of germ cell differentiation is limited (non-patent document 4). Therefore, whether hESCs/iPSCs are efficiently induced to the human germ cell fate is unknown; however, there are many reports teaching that random differentiation of hESCs/iPSCs produces germ cell-like cells with low efficiency (non-patent document 8).